The impact of rare earth element La on the microstructure and hot crack resistance of ADC12 alloy was analyzed. The additive amount of La was 0％, 0.3 wt％, 0.6 wt％ and 0.9 wt％, respectively. The results showed that, with the increase of the additive amount from 0％ to 0.6 wt％, the grain shape of α-Al gradually varied from developed dendritic crystal into fine dendritic crystal, equiaxed crystal and spheroidal crystal; eutectic silicon varied from needle-like or tabular shape into fine rod like shape; the hot crack force of the alloy also gradually decreased. However, when the additive amount of La reached 0.9 wt％, the excessive amount of rare earth elements was segregated within grain boundary area, forming intermetallic compounds. Therefore, the grain size of α-Al, eutectic silicon and the hot crack force of the alloy all increased. In the case that the additive amount of La reached 0.6 wt％, the best metamorphism effect and most excellent hot cracking resistance capacity of alloy were presented. The poisoning effect of rare earth element on eutectic silicon and the constitutional supercooling caused by rare earth element were the major causes for alloy modification, alloy refinement, and the main reasons for the increased hot cracking resistance.